This invention concerns a linearizing device for a frequency-modulation ramp. It applies to the field of radio altimeters, and more precisely to high linear and high precision radio altimeters.
It is well known that frequency modulation is used to generate linear frequency-modulation ramps which can be used in continuous wave (or CW) radio altimeters.
Such a radio altimeter is described, for example, in xe2x80x9cLes Techniques de l""lngxc3xa9nieurxe2x80x9d, E6601, page 11. This type of radio altimeter performs well at medium and low altitudes. However, it is generally admitted that only pulse radars provide sufficiently sensitive and accurate data at very high altitudes.
The factors which limit the performance of xe2x80x9cFM/CWxe2x80x9d radio altimeters at high altitude are:
the non-linearity of the signal modulation which results in a widening of the received signal spectrum.
the transmitter phase noise: the useful signal fades into the noise.
the transmission/reception coupling.
The main aim of the present invention is to extend the utilization range of frequency-modulation continuous wave radio altimeters to very high altitudes.
To achieve this, the invention features a device enabling to obtain a very high frequency-modulation linearity. The device uses a digitally controlled oscillator. This oscillator is also known as a xe2x80x9cDirect Digital Synthesizerxe2x80x9d hereafter referred to as a xe2x80x9cDDSxe2x80x9d.
An oscillator of this kind has the inherent advantages of digital technology, which are as follows:
reduction or elimination of any required adjustments,
good performance at high temperatures,
increased reliability,
frequency stability linked to that of the reference oscillator.
As has been shown, one of the conditions for obtaining a great accuracy from an altimeter is the use of a frequency-modulation ramp which is as linear as possible.
Current technology uses DDS""s consisting of a phase accumulator, a memory containing a conversion table and a digital/analog converter. The increase in the number of stages, i.e. the number of bits, naturally implies greater accuracy. These devices are also to be operated at high frequencies.
By way of non limiting example, DDS""s of this type are described in C. G. EKRC and S. I. LONG""S article xe2x80x9cA Ga As 4-bit Adder-Accumulator Circuit for Direct Digital Synthesisxe2x80x9d published in the xe2x80x9cIEEE Journal of Solid-State Circuitsxe2x80x9d, volume 23, No. 2, April 1988, pages 573-580 and in P. H. SAUL and D. G. TAYLOR""S article xe2x80x9cA High-Speed Direct Frequency Synthesizerxe2x80x9d published in the xe2x80x9cIEEE Journal of Solid-State Circuitsxe2x80x9d, volume 25, No. 1, April 1990, pages 215-219.
However, it should be mentioned that these circuits have very complex configurations, particularly as they operate at very high frequencies. Very fast technology (for example As Ga) and xe2x80x9cpipelinexe2x80x9d configurations must be used.
The invention aims to overcome the disadvantages of current technology whilst conserving the inherent advantages of digital circuits.
According to the present invention, there is provided a linearizing device for a frequency-modulation ramp comprising a voltage controlled oscillator fitted with an output and associated with a phase-locked loop, a clock generator having a determined frequency, a digitally controlled oscillator comprising a first accumulator controlled by a digital word for generating at least one digital step ramp varying in a determined direction at the rate of the determined frequency and a second accumulator fitted with a carry output and controlled by the digital ramp to supply a phase-variation digital step signal whose phase varies according to a parabolic law at the rate of the determined frequency and, inserted in the loop, a digital phase comparator having an output and a first and a second input respectively coupled to the carry output and to the output of the voltage controlled oscillator.
According to the present invention, there is further provided a radio altimeter comprising a transmitter for supplying a frequency-modulation ramp signal varying according to a linear law and a receiver for receiving the transmitted signal after reflection from a target at a determined distance resulting in a beat frequency between the transmitted signal and the received signal and a linearizing device of the ramp signal comprising a voltage controlled oscillator fitted with an output and associated with a phase-locked loop, a clock generator having a determined frequency, a digitally controlled oscillator comprising a first accumulator controlled by a digital word for generating at least one digital step ramp varying in a determined direction at the rate of the determined frequency and a second accumulator fitted with a carry output and controlled by the digital ramp to supply a digital step signal whose phase varies according to a parabolic law at the rate of the determined frequency and, inserted in the loop, a digital phase comparator having an output and a first and a second input respectively connected to the carry output and to the output of the voltage-controlled oscillator.